1. Field of the Invention
The present invention relates to an electrode structure including an electrical contact and to a method for making the structure, including attachment of the contact to the structure. In particular, the invention relates to an electrode structure for use in a color television pick-up tube and to a contact to be assembled with the electrode structure to provide a hermetically sealed connection to a transparent electrode on the interior surface of the front face of a television pick-up tube.
2. The Prior Art
In some types of television image pick-up tubes a transparent electrode is disposed on the inner glass surface of the faceplate of the tube, and an electrical contact extends through the glass face to connect with the transparent electrode. Such tubes are shown and described, for example, in U.S. Pat. Nos. 3,688,020 and 3,740,458. The faceplates of these tubes are multi-layer structures that include extremely thin and fragile glass plates, and there are two electrodes on the inner surface of the faceplate. The conductive means to carry signals through the tube wall to the electrode or electrodes is most conveniently arranged to pass through the faceplate, itself, rather than to be located on the cylindrical side wall or the base of the tube.
It is mandatory that the conductive means be hermetically sealed to the faceplate so that no air can enter the highly evacuated tube by any passageway alongside the conductive structure. The conductive structure must also make good electrical contact with the internal electrodes and must not include any substances that may interact deleteriously with any of the internal structure of the tube.
Various conductive means and methods of assembly exist for establishing a hermetically sealed electrical connection through a plate of glass or other transparent material to make contact with an electrode structure on the interior of an envelope of which the transparent plate forms one section. One such conductive means is a glazed contact wire inserted through a hole bored through an unpolished glass faceplate. The method of assembly is to heat the glazed contact wire to fuse it to the faceplate. Thereafter, the external surface of the plate is ground smooth. This grinding of the faceplate also includes grinding the wire off flush with the surface of the plate, but electrical contact can still be made to the exposed surface of the fused wire. A Nesa coating is applied to the other surface and is directly in contact with the contact wire.
Problems exist with such a method, however. The deposition process for a Nesa layer is strongly corrosive, and, therefore, the fused contact wire is likely to be chemically attacked. Furthermore, the corrosive Nesa layer can creep into the inner space between the glass and the fused wire and oxidize the surface of the wire to such an extent that the wire becomes unusable. Furthermore, such fused wire connections are expensive and often are not vacuum-tight.
Another method described in U.S. Pat. No. 3,675,062 allows a connection to be made to the interior conductive Nesa layer by drilling a hole through the glass faceplate and depositing the Nesa layer not only on the interior surface of the faceplate but also on at least a portion of the hole extending through it. Thereafter, an indium plug is inserted into the hole from the front side to make contact with the Nesa layer. In order to prevent the subsequent corrosion of the indium plug from the outside, it is necessary to cover the exterior end of the plug with a protective material such as a synthetic resin and then to use a device that resembles a sharp-pointed steel tack to pierce the resin and the end of the indium plug.
The latter structure and method are relatively complicated since the steps are not suited to automatic, machineoperated procedures. This results in poor quality control. Also, it is usually necessary to counter-sink the bore in the glass plate in order to provide suitable anchorage for the indium plug. However, grinding the faceplate to provide for counter-sinking may cause cracks to appear in the glass faceplate surface.
Another structure and method for providing a feed-through connection in a glass faceplate of a television pick-up tube are shown in U.S. Pat. No. 3,721,752 in which copper posts of cylindrical shape are inserted in holes in the faceplate and are sealed in place by indium bushings. The bushings also make electrical contact with electrodes previously deposited on the faceplate.
The latter structure gives rise to manufacturing problems, especially with respect to hermetic sealing.
U.S. Pat. No. 3,854,069 describes an improved contact structure and method of forming the same. The structure includes a post, the main portion of which is coated with a deformable metal, such as indium. The coated main portion of the metal post is inserted into a hole in the faceplate. A portion of the coating on the end of the metal post adjacent to the inner surface of the faceplate assembly is deformed to contact the electrode and to provide a hermetic seal against the electrode and the faceplate assembly interior surface. The enlarged head portion of the metal post is adjacent the exterior surface of the faceplate assembly, and in some embodiments the end of the metal post which is adjacent the interior surfaces of the faceplate assembly is covered with a photoconductive layer.
The process for coating the metal post with the deformable metal is to dip the metal post, which may be made of copper, into a heated bath which consists of a lower layer of liquid indium covered by an upper layer of non-volatilized glycerine or other type of alcohol compound having a high surface tension, and then quickly withdrawing the indiumcoated post. Prior to the coating the metal post is prepared by stripping away its oxide layer with an acid, such as hydrochloric acid. When the metal post is lifted from the bath the indium adheres to the post but due to the surface tension of the glycerine the indium coating on the post is tapered at its lower end. The orientation of the post is such that the lower end corresponds to the main portion of the post whose diameter is less than the diameter of the faceplate hole. Then the tapered lower end is inserted into a hole in the faceplate with sufficient pressure to deform the indium and seal the post hermetically to the faceplate.
However, the pressure required to deform the indium on the tapered post too often fractures the fragile glass faceplate.
It is therefore an object of the present invention to provide an electrical contact for use with an image signal pick-up tube which contact provides a hermetic seal with the faceplate of the image pick-up tube without placing excessive mechanical stress on the fragile faceplate.
It is another object of the invention to provide an electrical contact for use with the faceplate of an image signal pick-up tube wherein the contact is constructed to be free from defects due to corrosion.
It is still another object of the invention to provide an electrical contact for use with the faceplate of an image pick-up tube to make good electrical connection with an electrode on the interior surface of the faceplate.